This invention relates to hydraulic motors and pumps. For convenience of description however, reference is hereinafter made mainly to hydraulic motors.
Hydraulic motors are known comprising a casing having a plurality of hydraulic cylinders radially grouped about a floating bearing block, and a shaft journalled in said casing having an eccentric working in a cylindrical bore of the bearing block, the hydraulic cylinders being arranged to transmit liquid pressure to the bearing block imparting to it a circular motion with resultant rotation of either the eccentric with the motor shaft or rotation of the motor casing and hydraulic cylinders the shaft remaining stationary.
Generally, this type of hydraulic motor suffer two types of energy losses, namely mechanical friction loss and leakage loss of high pressure liquid, which decrease efficiency. Mechanical friction, mainly, sliding friction results in shortening the useful life of parts. Leakage of pressure liquid will cause a temperature elevation of the pressure liquid when it is recirculated for use. The temperature elevation of the working liquid will then cause a decrease in the viscosity of the liquid with the result of a further leakage.
If it is possible to apply hydrostatic bearing means to the sliding surfaces of the main parts of a hydraulic motor to keep each main part in a substantially perfect hydrostatic balance, this would result in a great decrease in the friction loss at mechanical pressure contact sliding parts and would also make it possible to decrease the leakage loss to the minimum. Thus, this solution of the problem is very effective to improve performance and obtain a hydraulic motor of highest efficiency adapted for high pressure running.
Attempts have been made to utilize hydraulic static bearing means between the plunger and the bearing block or between the plunger and the casing of hydraulic motors or pumps of the type described. For example, the hydraulic motor disclosed in U.S. Pat. No. 3,036,557 comprises a casing, a shaft, means journalling said shaft in said casing, an eccentric on said shaft, a bearing block within said casing and about said eccentric and having a bore permitting relative rotation between said bearing block and said eccentric, a plurality of hydraulic cylinders grouped radially about the axis of said shaft in a common plane normal to said axis, said cylinders being located in one of the members of the two comprising said casing and said bearing block, a hollow plunger in each cylinder and displaceable therein, resilient means urging said plunger in the direction of the other of said members, a pressure-retaining ring at the interface between said plunger and said other of said two members, said ring and the plunger end adjacent to said ring having openings in continuity with one another and with the plunger interior, and said bearing block having a port communicating between its bore and the volume defined by said plunger interior and said openings, whereby pressure liquid admitted to the cylinders in turn one after another during the operation of the motor exert direct thrust on the eccentric without the intermediary of mechanical means. The pressure retaining ring allows the pressure liquid to act over an area within the ring thus keeping the bearing block in balance and also provides for any misalignment between the surface of the bearing block and the inner end of the plunger.
In the hydraulic motor disclosed in the above mentioned U.S. patent, the pressure chamber for a hydrostatic bearing is formed either in the bearing block or in the interior surface of the casing. Since there is a relative sliding movement between the top or bottom end surface of the plunger and the cooperating interior surface of said casing or the cooperating surface of the bearing block, the pressure chamber for a hydrostatic bearing formed in the casing or the bearing block cannot be kept in alignment with the axis of the plunger. This produces a lateral moment which must be supported by the wall of the cylinder. Any pressure on the wall of the cylinder increases the friction between the plunger and the cylinder. On the other hand, in order to support by the cylinder the moment acting on the plunger the contact length of the plunger with the cylinder must be relatively long. A long contact length of the plunger is disadvantageous in that the sealing engagement between the top or bottom surface of the plunger and the cooperating surface of the casing or the bearing block is ready to be lost because any lateral movement of the plunger with respect to the cylinder is completely constrained by the cylinder.
On the other hand, recently there has been eager demand for a hydraulic motor which has superior performance in connection with starting and very-low speed operation, or presents a minimum difference between starting torque and running torque and which ensures smooth rotation without casing slip stick.
With the above in mind, the present invention is intended to provide a hydraulic motor or pump wherein the loaded sliding surfaces of the main parts are arranged to support loads in a hydrostatic bearing fashion to minimize contact surface pressure due to mechanical (solid-to-solid) contact between parts on loaded sliding surfaces, and friction loss to loaded sliding surfaces and particularly adverse effects due to coulombs friction are minimized and wear is also minimized, so that the motor or pump has a long life and is adapted for use at high pressures.
Another object of the invention is to provide a hydraulic motor or pump whose external dimensions are small as compared with its effective displacement (volume displaced).
A further object of the invention is to provide a hydraulic motor whose rotating parts have a small amount of moment of inertia as compared with the torque generated.
A still further object of the invention is to provide a hydraulic motor whose compressive volume is small as compared with the displacement.
An additional object of the invention is to provide a hydraulic motor or pump which is relatively small in the number of components, easy to manufacture and inexpensive.
Yet another object of the invention is to provide improvements in hydraulic motors and pumps thereby to simplify their construction and increase their efficiency.